The attorney general said the unit, first established nearly two decades ago in the aftermath of the Oklahoma City bombing, would form a collaboration across the Justice Department, to include the FBI, federal prosecutors and other agencies, to thwart potential domestic threats.

"As the nature of the threat we face evolves to include the possibility of individual radicalization via the internet – it is critical that we return our focus to potential extremists here at home,'' Holder said in a video message posted Monday morning on the Justice website.

"We face an escalating danger from self-radicalized individuals within our own borders,'' he said.

Domestic terrorists were responsible for more than two dozen terror incidents since the Sept. 11 attacks, including the deadly shooting at Fort Hood, Texas and last year's Boston Marathon bombings, which killed three and wounded more than 260 others, Holder said.

Holder said the unit was necessary to respond to the changing terrorist threat, notably the "reduced'' risk posed by al-Qaeda's core leadership. While significant threat remains from al-Qaeda affiliates in Yemen and Somalia, Holder said that the U.S. "must concern ourselves with a different type of threat.''

Holder said the unit, first organized under former Attorney General Janet Reno, was set to meet on the morning of Sept. 11, 2001, but the session never took place as the nature of the threat turned toward Afghanistan and Pakistan.